1. Field of the Invention
The present invention relates to an image-taking optical system which is used in image input apparatuses and the like typified by a document camera and an electronic magnifier to form an image of a two-dimensional object on an image-taking surface.
2. Description of the Related Art
In an image input apparatus having an image-taking optical system for forming an optical image of a two-dimensional object on an image pickup device (photoelectric converting element), the image-taking optical system is disposed at a position in the direction of the normal line of the center of the two-dimensional object as proposed, for example, in Japanese Patent Application Laid-Open No. 2000-235226. Thus, the image input apparatus has a post for fixing the position of the image-taking optical system and a stage for providing the apparatus with a stable center of gravity.
A known example of taking an image of an object obliquely thereto is a shift lens proposed in Japanese Patent Application Laid-Open No. 10-341319. This employs a wide-angle lens with a wide field angle as the image-taking optical system, and an image pickup device is disposed with a position shift from the optical axis of the image-taking optical system to take images by using the periphery of the field angle.
On the other hand, for a noncoaxial optical system, Japanese Patent Application Laid-Open No. 9-5650 discloses a design method thereof and a calculation method of a paraxial quantity such as a focal length, while Japanese Patent Application Laid-Open Nos. 8-292371, 8-292372, and 9-222561 disclose exemplary designs thereof. In the proposals in these applications, the concept of “a reference axis” is introduced and constituent surfaces are formed as asymmetric aspheric surfaces to allow provision of an optical system which has sufficiently corrected aberration.
Such a noncoaxial optical system is called an off-axial optical system. Specifically, it is defined as an optical system which, when a reference axis is defined as being along a light ray passing through the center of an image and the center of the pupil, includes a curved surface (off-axial surface), the normal line of which at the intersection of the reference axis and the constituent surface is not on the reference axis, and in this case the reference axis has a bent shape.
The off-axial optical system generally has noncoaxial constituent surfaces, and no vignetting occurs on a reflecting surface, so that it is easy to form an optical system using the reflecting surface.
Since the image input apparatus proposed in Japanese Patent Application Laid-Open No. 2000-235226 mentioned above has the image-taking optical system disposed at the position in the direction of the normal line in the center of the two-dimensional object, the apparatus requires a post extending from the side of the two-dimensional object for supporting the image-taking optical system and the stage for ensuring the stability of the entire apparatus for the position of the center of the gravity of the post. The entire apparatus thus tends to be increased in size.
FIG. 18 schematically shows a two-dimensional object and an image pickup device which are shifted from the optical axis of an image-taking optical system, which corresponds to the use of the aforementioned shift lens. This image input apparatus requires the use of a large lens system with a wide field angle which covers a field angle θ1 significantly larger than θ2, even though the actually used field angle is 2θ.
In a typical lens system, an amount of light is reduced as an angle with respect to an optical axis in the field is increased. Consequently, as a lens system with a wider field angle is used, a larger difference occurs in brightness between both ends of the image pickup device.
FIG. 19 schematically shows a configuration with an optical axis A directed to the center of a two-dimensional object. In a typical lens system, an image is not formed on an image pickup device I′ but on a plane I in parallel with the two-dimensional object. In such a configuration, the formed image is distorted in a trapezoidal shape and the object is out of focus at both ends of the image pickup device.
When the inclined image surface is corrected, it is necessary to cancel the difference between an optical path L1 of a light ray passing at the top of the two-dimensional object S and an optical path L2 of a light ray passing at the bottom of the two-dimensional object S (to bring the two-dimensional object S on a plane S′). This correction, however, is difficult to perform by simple oblique image-taking.
In addition, when a shift lens is used in the image-taking optical system for oblique image-taking which is included, for example, in the image input apparatus proposed in Japanese Patent Application Laid-Open No. 10-341319, the height of the image-taking optical system from a two-dimensional object is not greatly different from that when the image-taking optical system faces the two-dimensional object. In view of the need to hold the image-taking optical system at that height, it is unlikely that the apparatus is significantly reduced in size.